In-band Full-duplex Relay Research Articles

A Circularly Polarized In-Band Full Duplex Relay Antenna Solution for Smart Vehicular Traffic Management

A Circularly Polarized In-Band Full Duplex Relay Antenna Solution for Smart Vehicular Traffic Management

Impact of Outdated CSI on the Secure Communication in Untrusted In-Band Full-Duplex Relay Networks

To provide reliable connectivity in recent and future wireless communication systems, it is necessary to deploy several relay nodes. Further, a full-duplex (FD) technique has been in the spotlight since it can significantly improve spectral efficiency, and thus recent studies in relaying networks have considered FD relays. In relaying networks, confidentiality between the source and destination nodes from the relay node should be carefully kept since the relay node cannot be fully trusted, so called <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">untrusted</i> relay node. To this end, in this paper, we consider physical-layer security taking into account an untrusted FD relay node. We investigate a secure relaying protocol against the untrusted relay node where the destination generates artificial noise to prevent the untrusted relay from decoding the source information. We derive the analytical expression of the lower bound of the ergodic secrecy rate ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\bar {R}$ </tex-math></inline-formula> ). We find two main factors affecting the secrecy performance: residual self-interference (RSI) at the FD-available nodes (i.e., relay and destination), and outdated channel state information (CSI) at the destination. Thereafter, we evaluate their effects on <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\bar {R}$ </tex-math></inline-formula> and suggest the algorithm to find the sub-optimal artificial noise power level at the destination for maximizing <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\bar {R}$ </tex-math></inline-formula> . Through simulations, we have verified our mathematical derivation and shown that our secure relaying protocol can achieve near-optimal secrecy performance. Numerical results imply that the artificial noise power level should be carefully considered when the channel is severely outdated and RSI is irresistible.

On Performance of Full-Duplex Decode-and-Forward Relay Systems with an Optimal Power Setting under the Impact of Hardware Impairments

In this paper, we analyze the performance of in-band full-duplex (IBFD) relay systems that use the decode-and-forward (DF) protocol at the relay under the impact of imperfect self-interference cancellation and hardware impairments. Three practical relay scenarios are considered in our analysis: (i) there is no direct link from the source to the destination; (ii) there is a direct link, but the signal from the source is considered interference; and (iii) there is a direct link, and the signal from the source is cooperatively combined with that from the relaying path. Specifically, we derive exact and asymptotic expressions for the outage probability (OP) of the IBFD system. Based on the OP, the exact expression for symbol error probability (SEP) is also derived. Moreover, in order to cope with the effect of imperfect self-interference cancellation (SIC) due to the full-duplex mode, we propose optimal and suboptimal power calculation methods for the relay to minimize the OP and SEP. A performance evaluation shows that the IBFD relay system is significantly affected by both imperfect SIC and hardware impairments. However, the optimal power values can help to improve the system performance, significantly.

Compact Inband Full-Duplex Relays With Beyond 100 dB Self-Interference Suppression: Enabling Techniques and Field Measurements

In this communication, the self-interference (SI) channel and the novel enabling techniques for a compact inband full-duplex relay are described and characterized in different operating environments. The full-duplex operation is based on a novel antenna design that uses wavetraps to provide passive isolation of up to 70 dB between the transmit and receive antenna ports. The passive isolation is complemented with novel active RF and digital cancellation stages that further suppress the residual SI to the receiver noise floor. Measurement results of a complete prototype implementation show that the proposed design can achieve an overall SI cancellation performance of over 100 dB even with an ambitious instantaneous bandwidth of 80 MHz. Similar results are obtained both in an anechoic chamber as well as in realistic multipath indoor environments.

Design of Wavetraps for Isolation Improvement in Compact In-Band Full-Duplex Relay Antennas

In full-duplex (FDX) multiple-input and multiple-output (MIMO) systems, the self-interference between each transmitting and receiving antenna, caused by finite isolation between antennas, limits the obtainable signal-to-interference ratio of the system and thus the total capacity. In this paper, the design of planar wavetraps for isolation improvement between multiple antennas is introduced, with an emphasis on back-to-back relay structures. General guidelines for the design of planar wavetraps are deduced which could be applied also in isolation improvement of other antenna structures. A prototype back-to-back relay antenna with two dual-polarized patches for FDX MIMO is presented with the frequency of operation of 2.6 GHz. The prototype antenna is manufactured and the simulated results are validated with measurements. The minimum isolation among each antenna pair on the opposite sides of the relay is improved by 20 dB across a 18-MHz bandwidth, while 15-dB improvement can be obtained across 167-MHz bandwidth, yielding total isolation levels of 70 and 65 dB, respectively. The radiation pattern of the relay antenna is measured with and without wavetraps to show that the wavetraps do not have significant effect on the main lobe radiation properties.

Effects of antenna array characteristics on in-band full-duplex relays for broadband cellular communications

Abstract This paper considers a broadband multicell system with key characteristics that base-stations and relays employ beamforming via large antenna arrays, as well as in-band full duplex relay operation. The signal to interference plus noise ratio gain (which is in excess of the maximum required for such a system) could be exploited either for reducing self-interference or the backhaul and access link transmitted powers. The presented results indicate that with a moderate front-to-back-ratio scenario for the antenna array, “small” antenna arrays with 8 elements can lower the requirement of full duplex self-interference cancellation by up to 55 dB, while large antenna arrays of 1024 elements will produce 100 dB reduction. Alternatively, for the same scenario, both the backhaul and access transmitted powers could be reduced by ∼55 dB and ∼25 dB with 1024 elements.

Recent advances in antenna design and interference cancellation algorithms for in-band full duplex relays

In-band full-duplex relays transmit and receive simultaneously at the same center frequency, hence offering enhanced spectral efficiency for relay deployment. In order to deploy such full-duplex relays, it is necessary to efficiently mitigate the inherent self-interference stemming from the strong transmit signal coupling to the sensitive receive chain. In this article, we present novel state-of-the-art antenna solutions as well as digital self-interference cancellation algorithms for compact MIMO fullduplex relays, specifically targeted for reduced-cost deployments in local area networks. The presented antenna design builds on resonant wavetraps and is shown to provide passive isolations on the order of 60–70 dB. We also discuss and present advanced digital cancellation solutions, beyond classical linear processing, specifically tailored against nonlinear distortion of the power amplifier when operating close to saturation. Measured results from a complete demonstrator system, integrating antennas, RF cancellation, and nonlinear digital cancellation, are also presented, evidencing close to 100 dB of overall self-interference suppression. The reported results indicate that building and deploying compact full-duplex MIMO relays is already technologically feasible.